It does not sound like a ram issue to me. I'm not promising it's not the ram & it wouldn't do any harm to run memtest anyway just to be sure, but the symptoms you described are not typical of a faulty ram module.
It could be your hard disk as you suggested but if its the drive, I would expect you to have problems at other times, not just when moving the mouse after a period of inactivity.
The fact that it's happening mainly after a period of inactivity suggests your problem may be power management related.
It's still worth checking the hard disk though. The best diagnostic utility to use is usually the one provided by your HDD's manufacturer. Find out what make your drive is then go to the manufacturers website & download the relevant utility (or tell me what make it is & I'll give you the link to the correct web page).
Also can you tell me :
1) What your power management settings are in Windows (control panel->power options)
2) What make & model of motherboard you have. If you don't know, then download & run SWI to find out :http://www3.sympatico.ca/gtopala/
3) When you move the move the mouse after a period of inactivity & the system freezes with the HDD LED lit, is the LED already lit before you move the mouse or does it come on after you move it? Does using the keyboard first, instead of the mouse, in this situation make any difference to whether it freezes or not?